Knee replacement surgery is really a cartilage replacement. The knee itself is not replaced, only the damaged cartilage and bone ends. Knee replacement implants include a metal alloy on the end of the thighbone and polyethylene (plastic) on the top of the tibia and underneath the kneecap. The implant is designed to create a new, smoothly functioning joint that can prevent painful bone-on-bone contact. Your surgeon may elect to replace all or part of your knee, depending on your condition and the extent to which your knee is affected by arthritis.